1. Technical Field
The present invention relates to a development device and an image forming apparatus.
2. Related Art
In an image forming apparatus using non-magnetic one-component toner, the toner is charged with frictional electrification using a development roller. In order to efficiently generate frictional electrification, JP-A-2001-66876 discloses a development roller for allowing toner to be effectively rubbed against the development roller by performing a blast treatment with respect to the development roller and applying predetermined surface roughness Rz to the surface of the development roller.
However, the size, depth, shape and arrangement of the concave portions formed by the blast treatment are not uniform. Accordingly, since the toner which goes into a deep concave portion is not rolled, the toner may not be effectively charged. Filming may occur due to the unevenness of the irregularities of the surface of the development roller. If the toner is not effectively charged, toner may leak from a development device so as to be scattered in an image forming apparatus or fogging may occur in an image.
In order to improve the charging properties of the toner, JP-A-2007-121947 discloses a development roller, in which grooves are regularly arranged in a lattice shape by rolling, and a method of manufacturing the same. This development roller, in which the grooves are regularly formed, further improves the charging properties compared to a development roller having an irregular surface state by the blast treatment.
However, even in the development roller disclosed in JP-A-2007-121947, there is a filming with faint coloring. This filming is hereinafter referred to as “colored filming” because the filming grows as printing is repeated and the surface of the development roller is gradually appears to be dyed by the color of the toner. If the degree of colored filming becomes excessive, since the toner cannot be appropriately charged, the amount of scattered toner increases and fogging occurs in an image.
The cause of the colored filming is as follows. There are areas in which the toner cannot be sufficiently circulated in an inclined surface of a convex portion surrounded by the lattice-shaped grooves formed by rolling, and the toner is retained in those areas. When the temperature of the surface of the development roller increases during continuous printing, the retained toner is influenced by heat and filming gradually occurs. If the grooves are formed by rolling, a protrusion (hereinafter, referred to as “burr”) may occur in the ridge line of the convex portion surrounded by the grooves. In a portion which is the shade of a burr, the toner is apt to be retained and filming is apt to occur.
The toner which is not developed and is left on the development roller is typically removed by a feed roller, and new toner is fed such that the same toner does not continuously stay at the same place. However, if the burr is present in the ridge line of the convex portion, as shown in FIGS. 16A and 16B, the old toner cannot be completely removed, regardless of the rotation direction of the feed roller. Accordingly, the toner is retained in the vicinity of the burr. As a result, the retained toner causes filming.